title: Massive production of small RNAs from a non-coding region of Cauliflower mosaic virus in plant defense and viral counter-defense.
abstract: To successfully infect plants, viruses must counteract small RNA-based host defense responses. During infection of Arabidopsis, Cauliflower mosaic pararetrovirus (CaMV) is transcribed into pregenomic 35S and subgenomic 19S RNAs. The 35S RNA is both reverse transcribed and also used as an mRNA with highly structured 600 nt leader. We found that this leader region is transcribed into long sense- and antisense-RNAs and spawns a massive quantity of 21, 22 and 24 nt viral small RNAs (vsRNAs), comparable to the entire complement of host-encoded small-interfering RNAs and microRNAs. Leader-derived vsRNAs were detected bound to the Argonaute 1 (AGO1) effector protein, unlike vsRNAs from other viral regions. Only negligible amounts of leader-derived vsRNAs were bound to AGO4. Genetic evidence showed that all four Dicer-like (DCL) proteins mediate vsRNA biogenesis, whereas the RNA polymerases Pol IV, Pol V, RDR1, RDR2 and RDR6 are not required for this process. Surprisingly, CaMV titers were not increased in dcl1/2/3/4 quadruple mutants that accumulate only residual amounts of vsRNAs. Ectopic expression of CaMV leader vsRNAs from an attenuated geminivirus led to increased accumulation of this chimeric virus. Thus, massive production of leader-derived vsRNAs does not restrict viral replication but may serve as a decoy diverting the silencing machinery from viral promoter and coding regions.
authors.affiliation: Institute of Botany, University of Basel, Schönbeinstrasse 6, 4056 Basel, Switzerland.
authors.forename: Todd
authors.initials: T
authors.lastname: Blevins
authors.forename: Rajendran
authors.initials: R
authors.lastname: Rajeswaran
authors.forename: Michael
authors.initials: M
authors.lastname: Aregger
authors.forename: Basanta K
authors.initials: BK
authors.lastname: Borah
authors.forename: Mikhail
authors.initials: M
authors.lastname: Schepetilnikov
authors.forename: Loïc
authors.initials: L
authors.lastname: Baerlocher
authors.forename: Laurent
authors.initials: L
authors.lastname: Farinelli
authors.forename: Frederick
authors.initials: F
authors.lastname: Meins
authors.forename: Thomas
authors.initials: T
authors.lastname: Hohn
authors.forename: Mikhail M
authors.initials: MM
authors.lastname: Pooggin
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism / Q000821:virology*; D029681:Arabidopsis Proteins / Q000378:metabolism; D060565:Argonaute Proteins; D017796:Caulimovirus / Q000235:genetics* / Q000502:physiology; D004279:DNA, Viral / Q000096:biosynthesis; D009154:Mutation; D010935:Plant Diseases / Q000821:virology; D058727:RNA, Small Untranslated / Q000096:biosynthesis*; D012367:RNA, Viral / Q000096:biosynthesis*; D043244:Ribonuclease III / Q000235:genetics; D014779:Virus Replication
chemical_list: C117457:AGO1 protein, Arabidopsis; D029681:Arabidopsis Proteins; D060565:Argonaute Proteins; D004279:DNA, Viral; D058727:RNA, Small Untranslated; D012367:RNA, Viral; D043244:Ribonuclease III